1. Field of the Invention
This invention relates generally to the controlling of an electron beam in an image pick-up tube and, more particularly, is directed to the controlling of the electron beam in an image pick-up tube of the kind having a diode-type electron gun.
2. Description of the Prior Art
Generally, in photoconduction-type image pick-up tubes, such as Vidicons, a cathode electrode emits a beam of electrons of substantially constant current which is directed towards a target electrode having a photo-conversion layer deposited thereon. The electron beam from the cathode electrode is regulated by a first beam control or beam-forming electrode which converges or sharpens the electron beam, and by a second accelerating electrode which further sharpens or focuses the electron beam and accelerates the latter toward the target electrode. The electron beam from the cathode electrode is made to scan the target electrode and, in accordance with the amount of incident light from an object image projected on successively scanned elements of the target electrode, a video output signal is obtained from the latter. More particularly, as the electrons of the accelerated beam impinge upon successively scanned elements of the target electrode, more or less of the impinging electrons are absorbed by the target electrode to neutralize positive charges that reside thereon due to incident light from the object image projected on the target electrode. The varying amounts of electron-absorption provide an output current of corresponding intensities. However, when the amount of light incident on the target electrode of the image pick-up tube increases above a certain value, the constant current electron beam may not have sufficient charge to neutralize all of the positive charges stored at the various elements of the target electrode with a single scanning of the latter by the electron beam, that is, an electron beam shortage may occur. As a result of the failure of the electron beam to neutralize all of the charges on the target electrode, the video output signal will not precisely follow increases in the amount of incident light on the target electrode. Thus, as the amount of incident light increases, the video output signal derived from the target electrode decreasingly corresponds to the actual image viewed by the pick-up tube. Further, in the case of object images having a very high white level, the failure to neutralize all of the charge on the target electrode will cause a white after-image to be depicted by the video output signal even when the pick-up tube or television camera subsequently views another object image of relatively low white level.
If, in order to compensate for the foregoing problem, the current of the electron beam at the cathode electrode is pre-set to a relatively large value so as to avoid insufficient beam current for high levels of incident light, the focusing of the electron beam deteriorates so that the resulting video output signal has poor resolution in respect to the viewed object. The foregoing results from the fact that focusing of the electron beam is reduced with increasing current levels of the beam. Therefore, it is desirable to control the current level of the electron beam in accordance with variations in the level of incident light projected on the target electrode of the pick-up tube.
In one known arrangement for controlling the current level of the electron beam, a voltage corresponding to the video output signal is applied to the first or beam control electrode of the image pick-up tube to increase the current level of the electron beam for higher levels of incident light projected on the target electrode. Thus, the current of the electron beam is increased only for higher levels of incident light so as to avoid substantial deterioration of resolution at normal or lower levels of incident light. However, it has been found that, with the foregoing arrangement, when the voltage applied to the beam control electrode is of a low value, the video output signal does not accurately reflect or is indistinguishable for varying levels of incident light projected on the target electrode. Further, it has been found that the stability of the described arrangement is relatively poor, that is, the range within which the current level of the electron beam can be controlled in a stable manner is very narrow, and unwanted oscillation results.
In order to avoid the foregoing problems, it has been proposed in U.S. Pat. No. 4,322,662, dated Mar. 30, 1982, and having a common Assignee herewith, to provide an image pick-up tube having a diode-type electron gun with a beam current control circuit in which a resistor or relatively high resistance value is connected in series with the beam control electrode to provide an auto feed-back. In such arrangement, the control electrode current flows through the resistor of relatively high resistance, and the voltage drop across such resistor is used for obtaining a desired variation of control voltage. However, since the control signal source is of high impedance, as viewed from the control electrode of the image pick-up tube, such control signal source is liable to be adversely influenced by external noises. Further, many of the constituent devices that need to be associated with the image pick-up tube, such as, amplifiers and the like, have to be selected in consideration of the voltage drop across the resistor of high resistance value, with the result that a power source of relatively high voltage has to be provided.